Purpose. To determine if IRBP (interphotoreceptor retinoid-binding protein)
is damaged following irradiation by visible light in the presence of bound
all-trans retinal.
Methods. Following irradiation of the IRBP-all-trans retinal complex, the r
etinal was removed and damage to IRBP measured as loss of titratable thiol
groups, loss of tryptophan fluorescence, and changes in retinol-binding-ind
uced fluorescence.
Results. IRBP irradiated by itself showed only minimal loss of tryptophan f
luorescence; this loss was substantially increased by irradiation in the pr
esence of all-trans retinal. Thiol groups and retinol-binding activity were
also shown to be reduced. The damage to IRBP seemed to involve photosensit
ization by the all-trans retinal, which was in turn protected from bleachin
g by the IRBP. The binding affinity was shown to be reduced ten-fold follow
ing irradiation.
Conclusion. In the eye, IRBP can stabilise vitamin A and debatably may be r
esponsible for transport of different forms of vitamin A between the photor
eceptor cells and pigment epithelium. If this is the case, it would play a
key role in rhodopsin regeneration after bleaching. IRBP also appears to be
necessary to sustain photoreceptor cells. Light was shown to cause photose
nsitized damage to IRBP, and thus might impair the regeneration process and
photoreceptor viability.